European Patent Publication EP 0,813,321 A2 which claims the priority of two German Patent Publications DE 196 23 750 and DE 196 43 205, discloses a method for transmitting digital data impulses having two logic levels "0" and "1". The just mentioned publication teaches the provision of a data acquisition frequency with the help of a high frequency oscillator clock signal, whereby the high frequency clock signal is ascertained from an impulse duration. According to the known method it is possible to cause a data acquisition signal generator (DASG), the acquisition frequency of which is controllable in an open loop manner, to track the transmission frequency of a transmitted signal or to equalize any frequency tolerance fluctuations by means of simple RC-oscillators. On the one hand, the high frequency clock signals, simply referred to as clock signals or clock frequency are used for acquiring a time duration of an impulse from which the acquisition frequency is derived. On the other hand, the clock signals are also serving as a stepping frequency for advancing counters which generate, upon reaching of threshold values, the system and data acquisition frequency. The threshold values on their part may also be derived from the clock signals, for example as portions of the time duration of one impulse.
However, a permanent follow-up or intermediate synchronization is not possible with a conventional bit sequence of digital data impulses having unknown impulse levels. Thereby, it is conventionally necessary to repeatedly perform a new synchronization after a known number of data impulses, by using new synchronization impulses.
The disclosure of EP 0,813,321 also discloses a further development for pulse width modulated signals, whereby each data impulse is provided with its pulse flank change which in turn makes possible a follow-up synchronization through the data impulses. A transformation of digital data impulses into a pulse width modulation, however, is rather subject to a substantial effort and expense and is thus not feasible for all applications.
Moreover, encoding methods for digital data impulses are known in the art which involve encoding that permits clock signal recovery. Especially the Manchester encoding method involves recoverable clock signals. Manchester encoding combines timing or synchronizing signals with data signals.
European Patent Publication EP 0,178,622 A2 discloses a method for the clock signal recovery in connection with a Manchester encoded data transmission with a voltage controlled oscillator (VCO) including a frequency divider, selector means and a phase detector. European Patent Publication EP 0,185,556 A2 and European Patent Publication 0,425,302 A2 disclose methods for the clock signal recovery for Manchester encoded signals or for a digital closed loop phase decoder (Phasenregelschleifendecoder).
The Manchester encoding method is very advantageous due to its ability to recover clock signals. However, Manchester encoding cannot be employed in certain prior art methods that transmit digital data impulses to a data acquisition signal generator (DASG). In the Manchester encoding method the impulse flank change takes place within a data impulse width between an inverted impulse half and a non-inverted impulse half. A flank change may not take place at the beginning or end of an impulse width if Manchester encoding is to be applied. For the above mentioned conventional method it is necessary that the impulse flank change takes place at the beginning or at the end of a data impulse width for ascertaining the clock frequency from the time duration of an impulse width. Hence, in the above described conventional methods it is not possible to use Manchester encoding.